This invention relates to piezoelectric mechanical filters, and in particular, to the provision of novel multiple resonant mechanical filters using edge-mode vibration of a long or semi-infinite piezoelectric plate.
An electromechanical filter generally includes an input electromechanical transducer, an output electromechanical transducer and mechanical coupling means for transmitting vibration from the input transducer to the output transducer, and has been mainly used in MF applications such as IF filters. In many cases, one or more mechanical resonators are interposed between the input and output transducers, and are connected thereto by mechanical coupling means. Piezoelectric transducers are used for the input and output transducers in the piezoelectric mechanical filters.
As well known in the prior art, transducers and resonators in mechanical filters vibrate at a selected mode, for example, a contour mode of a square plate or a circular disk, flexural mode of a circular disk, a longitudinal mode of a rectangular plate or a bar, or a torsional mode of a column. Therefore, there are difficult problems in support of transducers and resonators and connection of lead wires thereto.
In the prior art, the support of resonators and transducers and the connection of electric leads thereto have been made by pressing metal terminal plates having a projection onto a portion corresponding to a node of the vibration, or by fixing lead wires and support wires at the portion of the node of the vibration by a solder agent.
The former has problems in reliability of registry of the projection with the node portion and in failure of contact therebetween. The latter has such faults that the fixing operation of the lead and support is difficult, and that resonant frequency is elevated by the deposited increased stiffness by solder agent.